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2 RAID Redundant Arrays of Inexpensive Disks Invented in by David Patterson and Randy Katz, UC Berkeley Now a multibillion dollar a year industry 2007 Matt Welsh Harvard University 2

3 RAID Motivation Speed of disks not matching that of other components Moore's Law: CPU speed doubles every 18 months SRAM speeds increasing by % a year In contrast, disk seek time only improving 7% a year Although greater density leads to improved transfer times once seek is done Emergence of PCs starting to drive down costs of disks (This is 1988 after all) PC-class disks were smaller, cheaper, and only marginally slower Matt Welsh Harvard University 3

5 Increasing Reliability Idea: Replicate data across multiple disks When a disk fails, lost information can be regenerated from the redundant data Simplest form: Mirroring (also called RAID 1 ) All data is mirrored across two disks Advantages? Disadvantages? 2007 Matt Welsh Harvard University 5

6 Increasing Reliability Idea: Replicate data across multiple disks When a disk fails, lost information can be regenerated from the redundant data Simplest form: Mirroring (also called RAID 1 ) All data is mirrored across two disks Advantages: Reads are faster, since both disks can be read in parallel Higher reliability (of course) Disadvantages: Writes are slightly slower, since OS must wait for both disks to do write This approach also doubles the cost of the storage system! 2007 Matt Welsh Harvard University 6

7 RAID 3 Rather than mirroring, use parity codes Given N bits {b 1, b 2,... b N }, the parity bit P is the bit {0,1} that yields an even number of 1 bits in the set {b 1, b 2,... b N, P} Idea: If any bit in {b 1, b 2,... b N } is lost, can use the remaining bits (plus P) to recover it. Where to store the parity codes? Add an extra check disk that stores parity bits for the data stored on the rest of the N disks Advantages: If a single disk fails, can easily recompute the lost data from the parity code Can use one parity disk for several data disks (reduces cost) Disadvantages: Each write to a block must update the corresponding parity block as well 2007 Matt Welsh Harvard University 7

13 RAID 3 Issues What is the MTTF of RAID? Both RAID 1 and RAID 3 tolerate the failure of a single disk As long as a second disk does not die while we are repairing the first failure, we are in good shape! So, what is the probability of a second disk failure? P(2 nd failure) = MTTR / (MTTF of one disk / # disks -1) This can be derived from independent and exponential failure rates See Patterson RAID paper for details 10 disks, MTTF (disk) = 1000 days, MTTR = 1 day P(2 nd failure) = 1 day / ( 1000 / 9 ) = What is the performance of RAID 3? Well, the check disk must be updated each time there is a write Problem: The check disk is then a performance bottleneck Only a single read/write can be done at once on the whole system! 2007 Matt Welsh Harvard University 13

15 Atomic Write Failure Many applications perform update in place They change a file on disk by overwriting it with a new version What happens with RAID? Disk 1 Disk 2 Disk 3 Disk 4 Check disk Matt Welsh Harvard University 15

17 Atomic Write Failure But is the complete write to all disks really atomic? Generally, no! What does this mean? Data can be left in an inconsistent state across the different disks! Really hard to recover from this. Problem: Most applications assume the storage system has atomic write semantics. Possible fixes? Use a journaling filesystem-like approach: Record changes to data objects transactionally. Requires extensive changes to filesystem sitting on top of the RAID. Battery-backed write cache: RAID controller remembers all writes in a battery-backed cache When recovery occurs, flush all writes out to the physical disks Doesn't solve the problem in general but gives you some insurance Matt Welsh Harvard University 17

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